The way I understand it.... and if I think about it intuitively, the leg with the high volatility should have a lower exposure to eliminate directional bias. i.e., ER2 (Russell 2K) has a higher volatility than ES (SP500) so a lower market exposure in ER2 vs. the ES exposure. Otherwise, you will introduce a directional bias.
BTW, I'm no expert in trading this spread, may be others here might have a better explanation.